1. Field of the Invention
The present invention relates to an automatic air conditioning system with a variable displacement compressor for automotive vehicles, particularly to an automatic air conditioning system with a variable displacement compressor for automotive vehicles, in which a temperature of conditioned air flowing through a lower discharge outlet, such as a foot vent, can be controlled at a higher temperature than a temperature of air flowing through another discharge outlet, such as a chest vent, in a BI-LEVEL mode in which two discharge outlets are simultaneously opened, and particularly in BI-LEVEL mode and fuel-saving mode in which the compressor is driven at a low speed and the discharge of refrigerant therefrom is relatively low.
2. Description of the Prior Disclosure
Recently, there have been proposed and developed various automatic air conditioning systems with a control unit which controls opening angle of doors, such as a switchable fresh/recirculation air intake door, an air mixing door, a defroster door, a chest vent door, a foot vent door, or the like, and controls the amount of air flowing through the evaporator of the air conditioning system in response to output signals from various sensors for detecting various physical quantities, such as an ambient temperature, room temperature in the vehicular cabin, magnitude of insolation, intake air temperature of the evaporator, suction pressure of the compressor, and in response to output signals indicative of ON/OFF state of various switches, such as an air conditioner switch, a blower switch, an ignition switch, a defroster switch, or so forth. In general, in a fuel-saving mode, such an automatic air conditioning system controls the compressor in such a manner that the discharge of the compressor is cut off at a low level, thereby resulting in the lowering of torque absorbed by the compressor. Therefore, it is required that a temperature of air flowing through the evaporator, which will be referred to as an "intake air temperature" is controlled so as not to become lower than the desired temperature. Conventionally, the intake air temperature is measured just behind the evaporator. In fuel-saving mode, it is not desirable that the cooled air flowing through the evaporator is reheated by the heater unit, for the reason of the wasteful consumption of a larger torque than actually necessary. In a conventional air conditioning system, in fuel-saving mode, the intake air temperature is so controlled as to be close to a target temperature of air to be discharged from discharge outlets, which will be referred to as a "target discharge air temperature". The target discharge air temperature is calculated by the CPU of the automatic air conditioning system on the basis of the various control parameters, such as a preset temperature manually input through a control panel mounted on an instrument panel of a vehicle, an ambient temperature detected by an ambient sensor, magnitude of insolation detected by an insolation sensor, room temperature in a vehicular cabin detected by a room temperature sensor. Therefore, in the fuel-saving mode, the air flowing through the evaporator is directly introduced through an air mixing door into an air mixing chamber because the air mixing door is so controlled as to be positioned in a fully closed position in which the air flowing through the evaporator is not introduced into the heater unit with the result that the air flowing through the evaporator is not partially reheated by the heater unit. Therefore, in the BI-LEVEL mode and the fuel-saving mode, a temperature of air discharged through an upper discharge outlet, such as a chest vent, and a defroster, is essentially equal to a temperature of air discharged through a lower discharge outlet, such as a foot vent, because the air flowing through the evaporator is not partially reheated by the heater unit. A feature of the BI-LEVEL mode is that conditioned air of two different temperatures is discharged through two different discharge outlets. In other words, in BI-LEVEL mode, the temperature of conditioned air flowing through the foot vent must be higher than that flowing through the chest vent or the defroster nozzle. However, in the prior art air conditioning system, the aforementioned effect of the BI-LEVEL mode is not satisfied in the fuel-saving mode.